1. Field of the Invention
The present invention relates to method and apparatus for testing blowout preventers and wellhead casing seals.
2. Description of the Prior Art
In completing offshore oil and gas wells it is necessary to install a wellhead and blowout preventer stack. The wellhead is installed by mounting it on the upper end of large diameter surface casing which has been lowered into a borehole and cemented in place. Because the casing string used to complete the remainder of the well will be of a smaller diameter than the surface casing used to install the wellhead, the wellhead is usually adapted to sealably support an inner concentric string of casing. A casing hanger, to which is affixed an inner string of casing, is lowered and landed within the bowl of the wellhead. The hanger, which has an enlarged diameter and is adapted to be supported by the wellhead bowl, is provided with a seal assembly around its outer periphery. The casing hanger is also adapted to land the next string of casing. Thus a nest of casing hangers can be employed to suspend successively smaller concentric strings of casing.
Secured on top of the wellhead is the blowout preventer stack. The BOP stack is usually an assemblage of one or more blowout preventers mounted together in a vertical arrangement. A combination of various types of preventers such as annular preventers, pipe rams, and shear rams is employed to provide adequate well safety, kick control, and blowout prevention under a wide range of working pressures and conditions.
To ensure that the wellhead casing hanger seals and blowout preventers provide the pressure integrity for which they were designed it is necessary to pressure test seal and blowout preventer. Almost all test procedures currently employed involve use of a drill string tool provided with an annular seal assembly. The seal assembly is lowered on a drill string until it engages and seats with the casing hanger bowl thereby closing off the upper end of the drill string. The blowout preventer to be tested is then closed around the drill string creating an annular chamber defined by the drill string, seal assembly, blowout preventer and wellhead. The annular chamber is then pressurized by introducing a fluid into the chamber through a kill or choke line. Detection of pressure leak off from around the wellhead casing seal or blowout preventer is then interpreted as indicating a faulty casing seal or preventer.
The main disadvantage of the above procedure is that leakage of a liquid across the casing seal or preventer is often at a slow, undetectable rate. Another technique, as exemplified and described in U.S. Pat. No. 3,093,996 (Jones), seeks to avoid this problem by employing a gas under moderate pressure as the testing fluid, the theory being that a leak which will readily permit the passage of gas will not necessarily leak liquid test fluid. However, such test procedures do not subject the seals in question to the high pressures which they must ultimately withstand. A seal which does not leak gas at 100 or 200 psi does not always indicate that the seal will function properly at 5000 psi.
A partial solution to the problem aof ascertaining leak detection is disclosed in U.S. Pat. No. 3,872,713 (Ilfrey et al). In that patent, an apparatus for testing wellhead casing seals employs a pressure sensing device positioned within a portion of a tubular test tool situated below the casing seal. Leakage of fluid past the casing seal increases the external pressure of the fluid surrounding the lower portion of tool causing the pressure sensing device to give a positive reading. Signals from the pressure sensing device are transmitted to the surface via a wireline cable. The above apparatus, however, only detects casing seal leaks and cannot indicate the pressure integrity of the blowout preventers.
The prior art therefore fails to provide a method or apparatus for rapidly and accurately detecting leakage of both a wellhead casing hanger seal and a blowout preventer.